Add environment variable to control local login availability

Add VITE_ENABLE_LOCAL_LOGIN env variable to conditionally show/hide
local username/password login on the login screen. When set to 'false'
(production), only Microsoft login is shown. When 'true' (development),
both options are available.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.env.development

@@ -11,3 +11,6 @@ VITE_WEBSOCKET_PATH=/socket.io/
 # MSAL Authentication (local development)
 VITE_MSAL_REDIRECT_URI=http://localhost:5173/
 VITE_MSAL_POST_LOGOUT_REDIRECT_URI=http://localhost:5173/
+
+# Local login (username/password) - enable for development
+VITE_ENABLE_LOCAL_LOGIN=true

.env.production

@@ -11,3 +11,6 @@ VITE_WEBSOCKET_PATH=/semblance_back/socket.io/
 # MSAL Authentication (production server)
 VITE_MSAL_REDIRECT_URI=https://ai-sandbox.oliver.solutions/semblance
 VITE_MSAL_POST_LOGOUT_REDIRECT_URI=https://ai-sandbox.oliver.solutions/semblance
+
+# Local login (username/password) - disable for production
+VITE_ENABLE_LOCAL_LOGIN=false

src/pages/Login.tsx

@@ -25,6 +25,9 @@ export default function Login() {
   const { login, loginWithMicrosoft, isAuthenticated, isMsalLoading } = useAuth();
   const [isLoading, setIsLoading] = useState(false);
 
+  // Check if local login is enabled (defaults to true for backwards compatibility)
+  const enableLocalLogin = import.meta.env.VITE_ENABLE_LOCAL_LOGIN !== 'false';
+  
   // Get the intended destination from state, or default to home page
   const from = location.state?.from || '/';
   
@@ -120,6 +123,8 @@ export default function Login() {
             </Button>
           </div>
 
+          {enableLocalLogin && (
+            <>
               {/* Divider */}
               <div className="relative mb-6">
                 <div className="absolute inset-0 flex items-center">
@@ -178,12 +183,16 @@ export default function Login() {
                   </Button>
                 </form>
               </Form>
+            </>
+          )}
         </CardContent>
         
         <CardFooter className="flex flex-col space-y-2">
+          {enableLocalLogin && (
             <div className="text-sm text-center text-gray-500 mb-2">
               Default account: user / pass
             </div>
+          )}
           {!isLoading && !isMsalLoading && (
             <div className="flex flex-col items-center justify-center gap-2">
               <Button